1) Field of the Invention
This invention relates to a tube type incandescent lamp having a structure in which short bars in a filament assembly are supported by their corresponding constricted portions formed in a tube envelope made of glass.
2) Description of Related Art
In tube type incandescent lamps used as the light sources for exposure by way of example, there has been a demand for a luminous intensity distribution that the light-emission intensity becomes higher at both outer portions thereof.
As a means for increasing the light-emission intensity at both outer portions of the tube type lamps, it has been known to date to use so-called differential light-emission type filament assembly in which coiled filaments and short bars are alternately arranged and both endmost ones of the coiled filaments are rendered longer.
On the other hand, in order to obtain a suitable luminous intensity distribution, it is necessary to situate a filament assembly along the tube axis of a glass-made tube envelope to hold it so as not to undergo displacement. As such a holding means, it has been known to provide constricted portions in the glass-made tube envelope so as to hold short bars by the constricted portions.
Now, a tube type incandescent lamp high in operation voltage requires a filament fine in wire diameter and a coiled filament formed of the fine-diameter filament is hence liable to be loose compared to that in a filament assembly used in a tube type incandescent lamp low in operation voltage. There has accordingly been a problem that when both endmost coiled filaments are rendered longer, they hang down due to their own weights, whereby a suitable luminous intensity distribution cannot be obtained.